Device for drying products such as in particular purifying station sludge

ABSTRACT

A device for drying sludge originating in a waste water treatment plant is enclosed in a greenhouse having a floor for receiving a bed of sludge to be dried. The greenhouse employs solar energy. A spreader located in the greenhouse ensures the the sludge is spread over the floor and turned as it progresses along the drying device. Fans are positioned over the spreader to provide for the renewal of air present in said greenhouse. A controller controls start up and shut down of a drying cycle as well as automatic control of all motorized components in response to measurement of the temperature of the surface of the bed of sludge. The controller also takes into account the difference in temperature between the surface of the bed of sludge to be dried and the atmosphere present in the drying plant, equipment in the greenhouse only being started up when this temperature difference reaches a predetermined set point.

FIELD OF THE INVENTION

The present invention relates to the drying of waste products, inparticular of sludge originating from treatment plants for thepurification of municipal or industrial wastewater, and it proposes tointroduce a drying device employing solar energy.

BACKGROUND OF THE INVENTION

It is known that sewage sludge is inevitably produced as a waste productduring the treatment of water: each individual produces on averageapproximately 20 kilos per annum thereof, which represents, for Europe,more than 10 million tons per annum, expressed on a dry basis, i.e. 50million tons of raw sludge when reference is made to “wet” sludge, whichis composed of 20% of dry mailer and 80% of water.

Currently, there are several possible end uses for this sludge, inparticular: disposal on a landfill site, incineration and landapplication: in each of these cases, the drying of the sludge turns outto be an essential stage which makes it possible to reduce by a factorof 4 the volumes to be stored, transported and disposed of. However, dueto numerous restrictions (in particular technical, health, regulatory,and the like), such routes for the discharge of sludge are increasingly,complex and thus increasingly expensive and they may even be challenged,indeed even banned, in places.

It is seen that this drying currently constitutes an expanding marketand that it represents a significant part of the resulting cost of theprocesses for disposing of/enhancing in value the waste productsrepresented by the biological sludge resulting from plants for thetreatment of wastewater. This part increases as a proportion of thebudget of the treatment plant as the capacity of the treatment plantdecreases.

The majority of plants for the drying of sludge are “thermal drying”plants. They consume large amounts of energy (approximately 1000 kWh perof water evaporated), in particular fossil fuels, and they require thepresence of qualified personnel and high capital costs. For this reason,these solutions according to the current state of the art are,economically speaking, poorly suited to small or medium capacity plants.

It has also been envisaged to dry the sludge by solar radiation, thistechnique exhibiting the advantage of using renewable energy and asimple construction technology coming under the notion of greenhouses ofhorticultural type. In order to improve the drying performance of suchsolar plants, the greenhouses are generally equipped with means whichprovide for the turning over of the sludge to be dried, indeed even itsprogression along the drying plant. Thus, the product to be dried alwaysexhibits a wet surface in contact with the air, thus preventing theformation of a “crust” at the surface of the bed of sludge and making itpossible to improve the efficiency of evaporation of the water duringthe drying treatment.

The solar dryer arrangements known currently provide for the extractionof the water vapor resulting from the gradual drying of the sludge byknown phenomena of natural convection induced by the differences indensity of the air in the greenhouse (these differences being due to thetemperature and humidity gradients); solar dryers of this type aresometimes equipped with fans in order to provide for forced circulationand forced replacement of the gaseous atmosphere of the dryer (forcedconvection).

In the known plants, these fans are situated in the top part of thegreenhouse (that is to say, “in the roofing”) and they are proportionedso as to provide a certain level of renewal of the total volume of theair present in the greenhouse. For this reason, the cost of the “forcedventilation” element represents between 25 and 50% of the electricityconsumption related to the operation of the dryer. In point of fact,optimization studies which have been carried out show that it is therenewal of the layer of air in immediate contact with the bed of sludgeto be dried which has the greatest repercussions on the efficiency ofthe evaporation process.

The proportioning of a solar drying device is related to the weatheraspects of the site where this device is installed. Furthermore, theoperability of the drying equipment is extremely limited during thenight and during winter: the annual availability, which is essentiallydiurnal, not exceeding 30% of the time. In addition, it is usual toregard, in temperate regions, 70% of the annual amount of water presentin the sludge being evaporated during the hottest 3 months of the year.

These various requirements make it necessary to significantly exaggeratethe size of the plants by providing large surface areas to dryrelatively low amounts of sludge. Depending on the regions where theplant is installed, on the one hand, and on the degree of optimizationof the processes employed, on the other hand, the surface area put intoa solar dryer oscillates between 0.3 and 1 m² of greenhouse per tonne ofsludge to be treated and per year (initial solids content of 25% andfinal solids content of 75%).

The operation of the solar drying equipment is advantageously under thecontrol of the variations in one or more parameters, such as, inparticular: the solar radiation measured, the temperature of the air orthe humidity of the air inside and outside the greenhouse.

The system is thus activated according to the drying capability of theair, without taking into account the water vapor partial pressure at thesurface of the sludge.

Another disadvantage of the solutions according to the prior art to thissolar drying is that the feeding of the sludge, at one of the ends ofthe dryer, is carried out using a mobile charging appliance of the“tracked tractor” type equipped with a mobile bucket. The sludge isdeposited in relatively even piles at the inlet of the greenhouse,without the surface of the sludge deposit being truly optimized andintegrated with the drying zone.

BRIEF DESCRIPTION OF THE INVENTION

Starting from the state of the art mentioned above, the presentinvention proposes to introduce a solar drying plant which makes itpossible in particular to reduce the drying time while expending lessenergy.

Consequently, the invention relates to a device for drying products,such as in particular sludge originating from plants for the treatmentof municipal or industrial wastewater, employing solar energy producedin the form of a greenhouse comprising a slab or floor on which the bedof sludge to be dried is deposited, means being provided for ensuringthat the sludge spread over said floor is turned over and that it isprogressed along the drying device, the latter additionally comprisingfans to provide for the renewal of the air present in said greenhouse,characterized in that said fans are positioned over the means whichensure that the bed of sludge is turned over and progressed.

According to another characteristic of the present invention, a systemfor sucking up and forcing back the air, positioned in the top part ofthe greenhouse, is additionally provided.

Other characteristics and advantages of the present invention willemerge from the description given below with reference to the appendeddrawings, which illustrate implementational examples thereof devoid ofany limiting nature. In these drawings:

FIG. 1 is a diagrammatic view in perspective of a drying deviceaccording to the present invention;

FIG. 2 is a sectional view, on an enlarged scale, through a verticalplane of the device of FIG. 1;

FIG. 3 illustrates another embodiment of the invention, in verticalsection, and

FIG. 4 is a view similar to FIG. 1 of another implementational exampleof the drying device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

Reference is made first to FIG. 1, in which the following have beenrepresented: by 1, the greenhouse of the drying device, which isinstalled on the slab 2 over which the bed of sludge 3 to be dried isspread. Low walls, such as 5, delimit at the sides the surface overwhich the bed of sludge is positioned. In this FIG. 1, the reference 4denotes the device which ensures that the bed of sludge 3 on the floor 2is turned over and progressed.

According to the invention, the fans 6 which ensure that the air presentin the greenhouse 1 is renewed are positioned on the motor-driven frame7 of the device 4, which is produced, for example, in the form of ascarification machine. By virtue of this arrangement, the renewal of theair in contact with the sludge to be dried is optimized: it isunderstood that only the air in contact with the bed of sludge 3captures the moisture from the sludge. The proximity of the fans to thebed 3 makes it possible to reduce the “limiting layer” resulting fromthe flow of the air above the surface of the bed 3. The fact ofproviding the fans 6 on the frame of the scarification machine 4 makesit possible, whatever the direction of progression of the scarificationmachine, for the ventilation always to be carried out on the newlyshifted product. According to the invention, an electricallimit-of-travel flipping system can be provided which makes it possibleto select the fans which have to be actuated in order to optimize theresults of the drying.

In addition to the advantage mentioned above, which consists inoptimizing the renewal of the air in contact with the sludge, theinvention makes it possible to accelerate the velocities of air incontact with the sludge, which makes it possible to reduce the dryingtime while expending less energy, the stream of air being focused on thedrying zone instead of being dispersed throughout the volume of thegreenhouse, as is the case in the drying systems according to the priorstate of the art.

Thus, the greenhouse into which air is blown at the rate of 30 000 m³/h,through an opening with an area of 35 m², gives a mean air velocity of0.24 m/s (excluding natural convection). The same flow rate, producedaccording to the invention close to the bed of sludge (the fans 6 beingpositioned at a distance of the order of 1 to 1.50 m from the surface ofthe latter), makes it possible to increase by 2 to 4 times the velocityof the air in contact with the wet surface of the bed of sludge.

It is understood that, in the device according to the invention, the airsweeps across the sludge as soon as the latter is mixed, the sludge thenbeing at a higher temperature than the ambient atmosphere (“black body”effect of the drying bed), which makes it possible to increase the vaporpressure differences between the turned-over sludge and the air. Therenewal of the air in the drying chamber is carried out by naturalconvection, which becomes more efficient as the air becomes laden withmoisture and therefore lighter.

According to the present invention, in order to ensure that the streamsof air in the greenhouse are managed in a controlled way, a system forsucking up and forcing back the air, represented diagrammatically by 8in FIG 1, which provides for continual renewal of the medium, can beprovided.

According to the invention, it is also possible to provide an additionalsystem for ventilation via the ground. This system is represented inFIG. 3, in which the reference 9 denotes a pipe for introducing airunder the effect of a fan 10, this pipe emerging through the bed ofsludge 3 via a geotextile membrane 11. This system is installed in thesecond half of the route which the sludge follows in the drying device,that is to say at a point where the sludge is already sufficientlydehydrated to exhibit a granular and therefore noncaking structure. Thefact, according to the invention, of injecting air through a geotextilemembrane makes it possible to rapidly remove a significant fraction ofthe interstitial moisture of the sludge, this moisture being immediatelyentrained by the sweeping of air induced by the fans 6 mounted on theframe of the scarification machine 4. The prototypes produced accordingto the present invention have made it possible to confirm that the airflow rates involved are low, of the order of 0.05 to 5 m³/m²/s, in orderto limit the air velocities to values below that of the velocity atwhich the dust is carried away.

According to the present invention, the operation (start up and shutdown) and the automatic control of all the motorized components(scarification machine 4, fans 6, 10 and means for sucking up andforcing back the air 8) can be correlated with the measurement of thetemperature of the surface of the bed of sludge 3 carried out usingmethods for long-range temperature recording, for example employinginfrared detectors. It is also possible to take into account thedifference in temperature between the surface of the drying bed and theatmosphere present in the drying greenhouse, the equipment only beingstarted up, for example, if this temperature difference is at least 3°C., that is to say:T=(T _(drying bed) −T _(atmosphere))≧+3° C.

According to the present invention, it is also possible to measure therelative humidity content of the atmosphere outside the dryinggreenhouse and to lay down a set point for bringing on line all themotorized components when this relative humidity is less than 80%, forexample.

Finally, it is also possible to couple the measurements of thetemperature difference T° C. specified above and of external relativehumidity so that the first of these two values which reaches apredetermined set point triggers the start up of the equipment of theplant.

In an alternative form illustrated by FIG. 4, provision has been madefor a system for feeding the sludge employing a metering pump 12 and asystem for distributing and dispersing the sludge 13, so as to optimizethe distribution of the sludge at the inlet of the drying device, thearea served being used as drying space. The distributing system 13 makesit possible to deposit the sludge as a uniform layer over the entirewidth of the greenhouse on the side of the area served, in a waycontrolled and automated according to requirements.

According to the present invention, it is also possible to employradiant panels, radiant pipes or any other system for heating largebuildings, in order to ensure that the surface of the bed of sludge isheated.

In the implementational example illustrated by FIG. 4, a radiant pipehas been represented by 14. The energy used to heat the pipe can be acombustible gas (introduced via a pipe such as 15) or biogas oralternatively a means for producing electricity from renewable ornonrenewable energy.

The use of this radiant heating technique exhibits the advantage ofheating the sludge by the effect of infrared radiation, the heat beingtransmitted essentially to the surface of the bed of sludge 3 withoutsubstantially heating the surrounding air. The volumes of air which aredisplaced by natural or forced convection will have a relatively slightaffect on the rise in temperature of the surface of the bed of sludge.The infrared radiation thus produced will make it possible to simulatethe 24 h/24 solar effect, whatever the period of the year, the weatherconditions, and the like. The surface area put into such drying devicescan, in this case, be reduced by a factor of 1.5 to 3.

It remains, of course, that the present invention is not limited to thevarious implementational examples described and/or represented here butthat it encompasses all the alternative forms thereof.

1. A device for drying sludge originating in a waste water treatmentplant, and comprising: a greenhouse having a floor for receiving a bedof sludge to be dried, the greenhouse employing solar energy; meanslocated in the greenhouse for ensuring that the sludge is spread oversaid floor and turned as it progresses along the drying device; fanspositioned over the spreading means to provide for the renewal of airpresent in said greenhouse; and means for controlling start up and shutdown of a drying cycle and automatic control of all motorized componentsin response to measurement of the temperature of the surface of the bedof sludge; wherein the controlling means also take into account thedifference in temperature between the surface of the bed of sludge to bedried and the atmosphere present in the drying plant, equipment in thegreenhouse only being started up when this temperature differencereaches a predetermined set point.
 2. A device for drying sludgeoriginating in a waste water treatment plant, and comprising: agreenhouse having a floor for receiving a bed of sludge to be dried, thegreenhouse employing solar energy; means located in the greenhouse forensuring that the sludge is spread over said floor and turned as itprogresses along the drying device; and fans positioned over thespreading means to provide for the renewal of air present in saidgreenhouse; means for controlling start up and shut down of a dryingcycle and of automatic control of all motorized components in responseto measurement of the temperature of the surface of the bed of sludge;wherein measurements of temperature difference and of relative humidityof the external atmosphere are coupled so that the first of these twovalues which reaches a predetermined set point triggers start up of theequipment.